Redox flow battery technology offers many advantages for grid energy storage such as load-leveling, long durability, flexible operation, easy scalability, high-efficiency and low cost. In this technology, electrochemical energy is stored in highly concentrated solutions of reversible redox active molecules, and separated in compartments for the low and high electrochemical potential species. Non-aqueous redox flow batteries (NRFBs) are a potentially viable alternative to their aqueous counterparts (ARFBs) having a wide range of redox active species and electrolytes available for their design. The energy density of NRFBs can be dramatically increased by using redox couples that are highly soluble in organic solvents and that operate at electrode potentials well beyond the window of stability of aqueous electrolytes. Despite these exciting prospects, the lower ionic conductivity observed in non-aqueous electrolytes has prevented the wide-scale development of NRFBs.
Challenges in adapting commonly used ion exchange membranes (IEMs) as separators from aqueous to non-aqueous environments are greatly responsible for the paucity in studies of NRFBs. The role of the separator is to physically and electronically isolate the high and low potential redox species compartments. This prevents the mixing of the redox active components (crossover) and simultaneously provides high electrolyte ionic conductivity for minimizing losses due to resistance to current flow. Using IEMs designed for aqueous environments, many of which are proton conductors, decreases the power density of NRFBs by one order of magnitude compared to ARFBs. Moreover, IEMs are expensive and they contribute to ˜20% of the battery cost.
Finding improvements in the performance of IEM's is an active research area, but an alternative membrane for NRFBs based on electrolyte size-selectivity rather than ionic-selectivity could be significantly beneficial. Size-selectivity using nano-porous membranes has been introduced recently in aqueous vanadium redox flow batteries for separating proton transport from that of larger vanadium cations. A strong emphasis is placed on the complex design of these membranes so they can adjust their sterics and electrostatics to effectively discriminate the redox active species, however this adds to the cost and complexity of the device. Accordingly, there is an ongoing need for new, more efficient, redox flow batteries. For example, an alternate approach in which the size of the redox active species is varied through a chemically-flexible synthetic polymer approach is needed in the art. Such a new strategy would de-emphasize membrane design and enable an insightful exploration of the properties of potential redox active candidates.